As materials for molding electronic parts, polyamide resins moldable into prescribed shapes by heat melting have been used in the past. In general, nylon 6, nylon 66, etc. have been widely used as the polyamides. Such aliphatic polyamides have excellent moldability but do not have sufficient heat resistance as raw materials for producing surface mount parts such as connectors that are exposed to such high temperatures as in a reflow soldering step. With such a background, nylon 46 has been developed as a polyamide having high heat resistance, but it has a problem of high water absorption. On that account, electrical and electronic parts molded by the use of a resin composition comprising nylon 46 suffer change in dimension because of water absorption, and if the molded article absorbs water, a problem of occurrence of blister, i.e., bulge, due to heating in the reflow soldering step is brought about. In contrast therewith, an aromatic polyamide derived from an aromatic dicarboxylic acid such as terephthalic acid and an aliphatic alkylene diamine has been developed. This aromatic polyamide has a feature that it not only has much more excellent heat resistance, mechanical strength and rigidity as compared with aliphatic polyamides such as nylon 66 and nylon 46 but also exhibits low water absorption.
Although polyamide resins are inherently self-extinguishing, they need to be blended with a flame retardant when they are used for surface mount parts requiring high incombustibility or flame resistance such as V-0 that is defined by Underwriters Laboratories Standard UL94. In general, it is publicly known technique that a compound referred to as a flame retardant assistant is properly added to allow a small amount of a flame retardant to exert a high flame retarding effect, in order to limit the amount of the flame retardant to a minimum. In the case of compositions having the same V-0, a composition containing a smaller amount of a flame retardant, that is, a composition containing a higher proportion of a polyamide resin, has an advantage of higher mechanical strength of the resulting molded article, such as toughness.
A system using a halogen compound as a flame retardant and using an antimony compound, such as antimony oxide or sodium antimonate, as a flame retardant assistant has been hitherto known to be a most typical flame retarding system satisfying the above requirement. In the case of the polyamide resins applied to the aforesaid uses, however, the molding temperature is generally high and not lower than 280° C. Consequently, in the presence of the antimony compound, decomposition or deterioration of the flame retardant, the polyamide resin, etc. is liable to take place, and troubles relating to occurrence of defectives, such as cleaning of a mold and discoloration of molded articles due to generation of a decomposition gas, are liable to occur. Thus, the polyamide resins are inferior in molding stability. In particular, polyamide resins having aromatic groups have high carbide-forming ability and have a problem of occurrence of discolored matters such as black spots.
In patent documents 1 to 5, polyamide compositions using an antimony compound and a non-antimony compound as flame retardant assistants are disclosed. By the use of such polyamide compositions, a flame retarding effect is obtained, but there reside problems in moldability, heat stability such as generation of a gas in the molding process and mechanical properties of molded articles obtained by molding the resins.
Patent document 1: Japanese Patent Laid-Open Publication No. H05-320503
Patent document 2: Japanese Patent Laid-Open Publication No. 265055/2000
Patent document 3: Japanese Patent Laid-Open Publication No. 501251/2001
Patent document 4: Japanese Patent Laid-Open Publication No. 506910/2002
Patent document 5: Japanese Patent Laid-Open Publication No. 128913/2003